1. Field of the Invention
The present invention relates to a webbing retractor. In particular, the present invention relates to a webbing retractor which, when pulling-out of a webbing is to be impeded, allows the webbing to be pulled out to a certain extent such that energy of an inertial force can be absorbed.
2. Description of the Related Art
In a webbing retractor, when the vehicle rapidly decelerates, rotation of a spool (a take-up shaft) in a direction of pulling-out a webbing is locked, such that pulling-out of the webbing is impeded. An example of the lock mechanism is a structure in which a lock device is disposed in a vicinity of a device frame at one end side of the spool. Due to the lock device operating when the vehicle rapidly decelerates, rotation of the spool in the webbing pull-out direction is impeded.
Further, in such a webbing retractor, at the time when pulling-out of the webbing is to be impeded, pulling-out of a predetermined amount of the webbing is allowed so as to absorb the energy of the inertial force of the vehicle occupant. An example of such an energy absorbing mechanism is a structure provided with, for example, a spool and a torsion bar which is coaxial with the spool. Generally, the torsion bar is connected to a shaft sensor, whose one end portion is connected to the spool and whose other end portion is connected to a lock device, such that the torsion bar, the shaft sensor, and the spool do not rotate relative to one another. Usually, the spool and the shaft sensor rotate integrally via the torsion bar. However, in a state in which rotation of the shaft sensor in the webbing pull-out direction is impeded when the vehicle rapidly decelerates, the spool rotates in the webbing pull-out direction with respect to the shaft sensor, due to the tensile force of the webbing. At this time, the torsion bar is twisted, the energy applied to the webbing is absorbed, and a predetermined amount of rotation of the spool is permitted. The absorbed energy is determined by the product of the load applied to the webbing (the force limiter load) and the webbing pull-out amount (the amount of rotation of the spool). In the webbing retractor, the force limiter load and the allowable amount of rotation of the spool (the twisting limit of the torsion bar) are given.
However, in such a conventional webbing retractor, the force limiter load at the time of energy absorption is a value which is determined exclusively by the values of the properties of the material of the torsion bar, and the dimensions and configuration of the torsion bar and the like. Only a value which is constant from the start of operation to the end thereof has been able to be used as the value of the force limiter load at the time of energy absorption.
In view of the aforementioned, an object of the present invention is to provide a webbing retractor in which, with a simple structure, a force limiter load can be changed at a predetermined timing, and the inertial energy of a vehicle occupant can be absorbed better.
The webbing retractor of the present invention comprises: a spool which is tubular and onto which a webbing is taken-up and from which a webbing is pulled-out; a shaft sensor provided at one end side of the spool, coaxially with the spool and so as to be able to rotate relative to the spool; a lock device connected to the shaft sensor, and when a predetermined acceleration is sensed, the lock device engages with a frame so as to impede rotation of the shaft sensor in a webbing pull-out direction; a torsion bar provided within the spool and coaxially with the spool, and one end of the torsion bar is connected to the spool and another end of the torsion bar is connected to the shaft sensor, and the torsion bar is usually rotated integrally with the shaft sensor, and in a state in which rotation of the shaft sensor in the webbing pull-out direction is impeded by the lock device, the torsion bar rotates the spool in the webbing pull-out direction relative to the shaft sensor, while being twisted due to a webbing tensile force; and a wire spanning between the shaft sensor and the spool and inserted movably within the spool, and in the state in which rotation of the shaft sensor in the webbing pull-out direction is impeded by the lock device, as the spool rotates until relative rotation of the spool with respect to the shaft sensor has reached a predetermined amount, the wire is rubbed at exit portion of the spool and imparts a rubbing force which offers resistance to the relative rotation, wherein the webbing retractor includes a force limiting device which, when pulling-out of the wire from the spool is being completed while the wire is being rubbed, limits the rubbing force at the time the pulling-out of the wire is being completed such that the rubbing force does not exceed rubbing force therebefore.
The wire encompasses, for example, rod-shaped members which do not easily deform, i.e., do not deform due to their own weight or by human force.
In the webbing retractor of the present invention, the spool and the shaft sensor are connected via the torsion bar. Usually, the spool, the shaft sensor and the torsion bar rotate integrally, and the webbing can be freely taken-up or pulled-out.
Here, when a predetermined acceleration (or deceleration) is sensed at the time when the vehicle rapidly decelerates such as, for example, the time of a vehicle collision or the like, the lock device operates such that rotation of the shaft sensor in the webbing pull-out direction is impeded. At this time, the webbing tensile force acts as torque, in the webbing pull-out direction, on the torsion bar via the spool. Thus, the torsion bar is twisted, and while the load applied to (the vehicle occupant via) the webbing is maintained constant (i.e., while a constant force limiter load is applied), the spool is rotated with respect to the shaft sensor in the webbing pull-out direction, and the webbing is pulled-out. The absorption of energy by the torsion bar is achieved.
When the torsion bar is twisted and the spool rotates relatively with respect to the shaft sensor, as the spool rotates, the wire is rubbed at the exit portion of the spool, and while being taken-up onto the side surface of the spool, imparts a load which offers resistance to this relative rotation, during the period of time until the relative rotation of the spool reaches a predetermined amount. Accordingly, in addition to the twisting load of the torsion bar, the rubbing force of the wire is applied to (the vehicle occupant via) the webbing as force limiter load. Namely, a force limiter load, which is greater than the force limiter load obtained only by the twisting load of the torsion bar, is obtained. The webbing pull-out speed (i.e., the rotational speed of the spool) can be kept low, and appropriate energy absorption can be achieved.
Further, when the spool rotates by a predetermined amount with respect to the shaft sensor, the entire wire, which spans between the shaft sensor and the spool, is pulled-out completely from the spool (i.e., the entire length of the wire is taken-up), and the imparting of load due to the rubbing of the wire is cancelled. Thus, only the twisting load of the torsion bar is applied to (the vehicle occupant via) the webbing as force limiter load. Namely, the force limiter load becomes lower than the initial force limiter load at the time when the energy absorption started.
In this way, in the webbing retractor of the present invention, the force limiter load can be changed at a predetermined timing. Namely, in the initial stages of rapid deceleration of a vehicle, the rubbing force due to the wire is added such that the energy absorption amount per unit time becomes large, and the pulled-out amount of the webbing (the amount of movement of the vehicle occupant) is suppressed. After a predetermined absorption of energy, by decreasing the energy absorption amount per unit time, the load applied to the vehicle occupant can be decreased. This is a desirable characteristic for the webbing retractor. In particular, in vehicles which are equipped with an air bag device, by reducing the force limiter load immediately before the air bag and the vehicle occupant contact, it is possible to reduce the load applied to the vehicle occupant, and injury to the vehicle occupant can be reduced further. Moreover, even in vehicles which are not equipped with an air bag device, by reducing the force limiter load immediately before the vehicle occupant and a portion within the vehicle, such as a steering wheel, an instrument panel (a dashboard), or the like, contact, it is possible to reduce the load applied to the vehicle occupant. This characteristic leads to a reduction in injury to the vehicle occupant.
Here, the webbing retractor of the present invention is provided with a force limiting device which limits the rubbing force, at the time when the wire is being rubbed while pulling-out of the wire from the spool is being completed, such that this rubbing force in the stages of completion of pulling-out does not exceed the rubbing force there before. Thus, the rubbing force which is imparted due to the rubbing of the wire does not suddenly increase at the time when the wire is completely pulled out from the spool. In other words, no large pull-out load of the wire arises. Accordingly, the force limiter load can be smoothly changed from a large value in the initial stages to a small value in the latter stages. In other words, the state is changed from a state in which the energy absorption amount per unit time is large to a state in which the energy absorption amount per unit time is small.
As described above, in the webbing retractor relating to the present invention, with a simple structure, the force limiter load can be changed at a predetermined timing, and the inertial energy of the vehicle occupant can be absorbed better.
In the webbing retractor of the present invention, usually, the force limiting device is a through-hole which is provided in the spool and in which the wire is movably inserted, the through-hole being formed such that a spool exit portion side of the through-hole has an enlarged diameter.
In the webbing retractor of the present invention, usually, the wire is inserted through the through-hole, and pulling-out of the wire from the spool is completed while the wire is being rubbed by the spool exit portion side of the through-hole (the peripheral portion of the through-hole). At this time, the spool exit portion side of the through-hole is formed to have an enlarged diameter. Thus, when pulling-out, from the through-hole, of the distal end portion of the wire, which is bent while being rubbed by (the peripheral portion of) the through-hole, is being completed (i.e., due to the distal end portion of the wire reaching the enlarged diameter portion of the through-hole), the radius of curvature of the distal end portion increases. Accordingly, the rubbing force imparted due to the rubbing of the wire does not increase suddenly at the time when the pulling-out of the wire from the spool is completed. In other words, no large pull-out load of the wire arises. Accordingly, the force limiter load can be smoothly changed from a large value in the initial stages to a small value in the latter stages. In other words, the state can be changed from a state in which the energy absorption amount per unit time is large to a state in which the energy absorption amount per unit time is small.
In the webbing retractor of the present invention, usually, the force limiting device is a taper portion provided at a wire distal end portion and formed so as to taper gradually.
Namely, in the webbing retractor of the present invention, usually, a taper portion, which serves as the force limiting device and which is formed so as to gradually taper, is provided at the distal end portion of the wire. Thus, at the time when the pulling-out of the taper portion of the wire, which is being pulled out from the spool while being rubbed, is being completed (i.e., due to the taper portion reaching the position of rubbing), the rubbing force decreases. Namely, the rubbing force does not suddenly increase when the pulling-out of the wire from the spool is completed. In other words, no large pull-out load of the wire arises. Accordingly, the force limiter load can be smoothly changed from a large value in the initial stages to a small value in the latter stages.
In the webbing retractor of the present invention, usually, the force limiting device is a heat-treated portion provided at a wire distal end portion, the heat-treated portion having a different rubbing force characteristic than other portions of the wire due to the heat-treated portion having been subjected to a heat treatment.
Namely, usually, in the webbing retractor of the present invention, a heat-treated portion, which serves as the force limiting device and whose rubbing force characteristic is different from that of the other portions due to having been heat-treated, is provided at the distal end portion of the wire. Thus, at the time when the pulling-out of the heat-treated portion of the wire, which is pulled out from the spool while being rubbed, is being completed (i.e., due to the heat-treated portion reaching the position of rubbing), the rubbing force decreases. Namely, the rubbing force does not suddenly increase when the pulling-out of the wire from the spool is completed. In other words, no large pull-out load of the wire arises. Accordingly, the force limiter load can be smoothly changed from a large value in the initial stages to a small value in the latter stages.
The webbing retractor of the present invention usually further comprises a delaying device which offsets, in time, a peak of an imparted load at a time when imparting of the rubbing force by the wire starts, with respect to a peak of a twisting load at a time when the torsion bar is twisted.
Namely, usually, in the webbing retractor of the present invention, the peak of the imparted load at the time when imparting of the rubbing force due to the wire starts, is offset, in time, with respect to the peak of the twisting load at the time the torsion bar is twisted. Namely, the peak of the rubbing force imparted by the wire, and the peak of the twisting load imparted by the torsion bar, arise so as to be offset from one another. The peak of the twisting load due to the torsion bar and the peak of the rubbing force due to the wire do not arise so as to be superposed on one another at a same time.
Accordingly, when the lock device operates and rotation of the shaft sensor in the webbing pull-out direction is impeded and the torsion bar is twisted (the force limiter load is applied) and the spool begins to rotate in the webbing pull-out direction with respect to the shaft sensor, the force limiter load (the twisting load due to the torsion bar and the rubbing force due to the wire) does not suddenly increase (no load having a large peak arises). It is possible to make the force limiter load smoothly rise to a predetermined value, and to apply the force limiter load.
In the webbing retractor of the present invention, usually, the delaying device is a through-hole provided in the spool and in which the wire is movably inserted, the through-hole being formed such that a diameter of a spool exit portion side of the through-hole increases at least toward an inner side of curvature of rubbing of the wire.
Namely, in the webbing retractor of the present invention, usually, the wire is inserted through the through-hole, and the pulling-out of the wire from the spool is completed while the wire is being rubbed by the spool exit portion side of the through-hole (the peripheral portion of the through-hole). Here, the diameter of the spool exit portion side of the through-hole is formed to be enlarged at least toward the inner side of the curvature of the rubbing. Thus, at the point in time when the wire is first rubbed (pulled) by (the peripheral portion of) the through-hole, hardly any rubbing force arises at the portion at which the through-hole is formed to have a larger diameter (during the time until the peripheral portion of the through-hole is reached). Namely, at the point in time when the wire is initially rubbed (pulled), rubbing force is first generated when the wire reaches the peripheral portion of the through-hole after having moved through the enlarged diameter portion of the through-hole. In this way, as a result, the peak of the imparted load at the time when imparting of the rubbing force due to the wire is started, is offset in time with respect to the peak of the twisting load at the time when the torsion bar is twisted. Namely, a peak of the rubbing force imparted by the wire, and a peak of the twisting load imparted by the torsion bar, arise so as to be offset from one another. The peak of the twisting load due to the torsion bar and the peak of the rubbing force due to the wire do not arise so as to be superposed on one another at a same time.
Accordingly, when the torsion bar is twisted (the force limiter load is applied) and the spool begins to be rotated in the webbing pull-out direction with respect to the shaft sensor, the force limiter load (the twisting load due to the torsion bar and the rubbing force due to the wire) does not suddenly increase (no load having a large peak arises). It is possible to make the force limiter load smoothly rise to a predetermined value, and to apply the force limiter load.